GNAT errormessage question

[haug@zesi.ruhrREMOVEIT.de (Haug Buerger)]

Dyn_Array_Test.adb:9:09: instantiation error at t.ads:14
Dyn_Array_Test.adb:9:09: controlled type must be declared at the library level

What is the meaning of this error message? What is
meant with library level?

Thanks
		Haug



PS: The source that produced the error.



with Ada.Finalization;
generic
	type Elem_Type is private;		
  	Bound: in Positive;	
package t is
	type Dynamic_Array is tagged private;
private
   type Dynamic_Array is new Ada.Finalization.Controlled with
   record
   	Elements	: Natural := 0;
   end record;
end t;



with Ada.Strings.Unbounded;
use Ada.Strings.Unbounded;
with t;

procedure Dyn_Array_Test is

   procedure Test_Dyn_Array is
   	package Dyn_Ustr_Array is new t ( Unbounded_String,Bound=>1000 );
   	use Dyn_Ustr_Array;
   	da: Dynamic_Array;
   begin
     	null;
   end Test_Dyn_Array;

begin
	Test_Dyn_Array1;
end Dyn_Array_Test;